Storing coded information on a surface by means of different types of marks is already known.
U.S. Pat. No. 5,852,434 describes, for example, a position-coding pattern which codes X-Y-coordinates for a number of positions on a writing surface. The position-coding pattern makes it possible for a user to record electronically graphic information which is created on a writing surface by continuously reading the position-coding pattern.
Three examples of the construction of the position-coding pattern are given in U.S. Pat. No. 5,852,434. In the first example the pattern consists of symbols, each of which is constructed of three concentric circles. The outer circle represents the X-coordinate and the middle circle the Y-coordinate. Both the outer circles are additionally divided into 16 parts which, depending upon whether they are filled in or not, indicate different numbers. This means that each pair of coordinates X, Y is coded by a complex symbol with a particular appearance.
In the second example, the coordinates of each point on the writing surface are given by means of bar-codes, a bar-code for the X-coordinate being shown above a bar-code for the Y-coordinate.
A checkered pattern which can be used to code the X- and Y-coordinates is given as a third example. However, there is no explanation as to how the checkered pattern is constructed or how it can be converted into coordinates.
A problem with the known pattern is that it is constructed of complex symbols and the smaller these symbols are made, the more difficult it is to produce the patterned writing surface and the greater the risk of incorrect position determinations, while the larger the symbols are made, the poorer the position resolution becomes.
A further problem is that the processing of the detected position-coding pattern becomes rather complicated, due to the fact that a processor has to interpret complex symbols.
An additional problem is that the detector or sensor which is to record the position-coding pattern must be constructed in such a way that it can record four symbols at the same time so that it is certain to cover at least one symbol in its entirety, which is necessary in order for the position determination to be able to be carried out. The ratio between the required sensor surface and the surface of the position-coding pattern which defines a position is thus large.
In EP 0 578 692 a position-coding pattern is described which is constructed of cells in the form of squares. The value of the cells is determined by their appearance, for example their color. The cells can be separated by separation zones so that two adjacent cells with the same color can be distinguished. The position-coding pattern differs from that according to U.S. Pat. No. 5,852,434 in that a particular number of cells, that is symbols, together code a position. The coding is in addition floating, which means that an arbitrary partial surface of the pattern which contains the above-mentioned number of cells codes a position. Each cell thus contributes to the coding of several positions. In this way the ratio between the required sensor surface and the part of the position-coding pattern which defines a position is less than in the above-mentioned US patent. In addition each cell is less complex and therefore the processor which is to decode the position-coding pattern needs to be able to recognize fewer different elements. However, the processor needs to be able to locate and distinguish at least two different cells.
EP 0 171 284 B1 shows another floating position-coding pattern which is constructed of horizontal and vertical lines, the vertical lines coding the position in the horizontal direction and the horizontal lines coding the position in the vertical direction. The lines can be found in positions which are a multiple of 1 mm. The presence of a line in such a position codes a one (1), the absence of a line in such a position codes a zero (0).
It is, however, difficult to record and decode a pattern of lines, as the intersections between the lines can be difficult to record. In addition, it is often the case that the sensor is not held parallel to the base, which results in a perspective where the lines are no longer parallel. It can then be difficult to determine whether a line is actually missing. In addition, there must not be too many missing consecutive lines, as difficulties can then arise in the decoding. Furthermore, the information content is small.
Applicant's Swedish Patent Application SE 9901954-9, which was filed on 28 May 1999 and which was not publicly available at the time of filing the present application and thereby does not constitute prior art, describes an additional position-coding pattern of the floating type in which the position information is coded graphically by means of dots of a first and a second size, a dot of the first size corresponding to a zero (0) and a dot of the second size corresponding to a one (1). A plurality of dots together code the coordinates for a position.
It is a general desire that coding patterns which are used to store information on a surface must be able to code a lot of information per unit area and must be simple to detect and decode even when subjected to interference of difference kinds.